Three-phase electrical motors are powered electrically by power converters, also known as inverters, which transform DC voltage, generally originating from an onboard battery, into AC voltage for each phase of the motor.
In other cases, the power converters modify an AC or DC input voltage into an AC or DC output voltage respectively having an increased or decreased voltage.
In certain applications, a plurality of power converters can be used in parallel to provide increased power.
In that case, asynchronous control signals can be used with pulse-width modulation. The control signals are termed asynchronous because their frequency does not depend on rotation frequency of the electrical motor supplied by the converters.
Examples of the prior art of such control signals are Toggle-type control signals or H3-type control signals. Toggle-type control signals allow startup losses to be minimized, while H3-type control signals are optimized for operation of the motors above 40% of their maximum rotational speed.
According to the prior state of the art, the asynchronous PWM control signals of each phase of the converters are derived from in-phase carrier signals.
The PWM H3 and Toggle signals are intersecting PWM signals, i.e. PWM signals resulting from the intersection of a modulating signal, representing the desired signal by a carrier signal of the sawtooth signal type.
FIG. 1 illustrates the modulating signal of a PWM H3 signal for a phase of a power converter and its components; FIG. 2 shows the generation of the PWM signal by intersection of the modulating signal and the carrier signals; and FIG. 3 shows the PWM signal thus obtained.
As shown in FIG. 1, the modulating signal of the PWM H3 signal marked 1 is derived from the sum of a fundamental sinusoidal signal at the frequency Fpwm marked 2 and a third-harmonic signal also of the sinusoidal type at the frequency Fpwm/3 marked 3. The amplitude selected in this example for the third-harmonic signal is equal to ⅙th of the amplitude of the fundamental signal. However; other amplitudes may also be selected.
FIG. 2 shows the intersection of the modulating signal 1 with a first sawtooth carrier signal 4a at the frequency Fpwm for the positive amplitudes, and with a second sawtooth carrier signal 4b at the frequency Fpwm for the negative amplitudes, where the two carrier signals are synchronous. FIG. 3 shows the PWM signal resulting from this intersection.
The control signal PWM H3 has the advantage of an output power of the power converter receiving the PWM signal generated from this modulating signal that is greater than the power of a power converter controlled by a PWM signal according to the prior state of the art.
FIG. 4 illustrates the modulating signal of a PWM Toggle signal for a phase of a power converter and its components; FIG. 5 shows the generation of the PWM signal by intersection of the modulating signal and the carrier signals; and FIG. 6 shows the PWM signal thus Obtained.
As shown in FIG. 4, the modulating signal of the PWM Toggle signal marked 6 is derived from the sum of a sinusoidal signal at the frequency Fpwm marked 7 and a square-wave signal at the frequency Fpwm marked 8.
In the example given, the amplitude of the square-wave signal is 0.5. However, other amplitudes may also be used.
FIG. 5 shows the intersection of the modulating signal 6 of the PWM Toggle signal with a first sawtooth carrier signal marked 9a at the frequency Fpwm for the positive amplitudes, and with a second sawtooth carrier signal 9b, also at the frequency Fpwm, for the negative amplitudes, where the two carrier signals are in phase opposition. FIG. 6 shows the PWM signal marked 10 resulting from this intersection.
However, the use of asynchronous control signals to control parallel power converters can generate specific, potentially negative effects such as the appearance of differential harmonics and zero-sequence harmonics.
In the current state of the art, such effects can be minimized only by using massive inductors and an interphase transformer reactor (ITR). This results in substantially large dimensions and weight of the power conversion system as a whole.
There is thus a need for more efficient power converter control in order to reduce the size and weight of the inductors and the interphase transformer reactor (ITR).